1. Field of the Invention
This invention relates to motion-producing devices within the broad category of prime movers. More particularly, this invention relates to positioning devices utilizing an elongate element the temperature of which is controlled to cause the element to contract and expand so as to develop an output motion.
2. Description of the Prior Art
Copending application Ser. No. 2,120, now abandoned, filed by E. O. Olsen on Jan. 12, 1970, discloses several positioning devices of a type to which the present invention is directed. In such devices, the motion-producing element is a wire illustratively made of Nitinol (a compound of Nickel and Titanium). Such material undergoes a temperature-responsive martensitic transformation which causes an internally generated contraction when the temperature is raised through the operating range. The wire is maintained continuously under tension developed by an external spring, and when the wire is cooled down through the operating temperature range, the spring force elongates the wire back to its original length. A feedback system, responsive to the position of the output member, automatically adjusts the heat input to the wire to maintain the output position in correspondence to an input signal. Such devices can be used for many purposes, such as positioning the pen of a chart recorder, actuating a valve, etc.